ROLE OF ANISOTROPY IN THE VORTEX LATTICE OF SUPERCONDUCTORS HIGH-Q OSCILLATOR MEASUREMENTS IN 2H-NBSE2 AND LA1.825SR0.075CUO4

Measurements of the response of the flux-line lattice in NbSe2 and La1 .825Sr0.075CuO4 show important differences between these two materials . In particular, we have studied the magnetic-field and angular depend ence of the response of a high-Q mechanical oscillator in fields of up to 1 T. The features seen in NbSe2 seem to be well explained in terms of a change in the pinning regime, usually termed the ''peak effect'' in the critical current, using the collective-pinning model of Larkin and Ovchivnikov within Ginzburg-Landau anisotropic theory. On the oth er hand the behavior found in the high-T(c) material LSCO seems to fal l naturally into a description which takes into account the possibilit y of phase transitions in the vortex lattice and the quasi-two-dimensi onal character of the superconductivity.